Missions Not Loading

You and your friend with both need copies of the game, along with installations of the Lucas' Simpsons Hit & Run Multiplayer launcher. I believe the regular launcher is also required but I could be wrong. Both players need to open the launcher, complete any setup needed, and then launch with the same mods and mutators enabled. Mutators can be adjusted on the Multiplayer launcher's "Mutators" tab, while a mod can be enabled in Multiplayer by dragging the mod's LMLM file onto the Multiplayer launcher's EXE file (some mods will be buggy if not designed with Multiplayer compatibility in mind).

Your friend will also need an account on this website in order to validate and access the Multiplayer.

i'm gonna try man, but no appears the tab of multiplayer

man, it won't let me play and we have everything the same in the launcher, when I enter the game nothing multiplayer appears or anything like that.

You need to launch from the Multiplayer launcher by the way.

ie do I need to open both launchers?

No, you only need to open and launch from the Multiplayer launcher.

but which of the two is it?
Launcher
Lucas Simpson Hit And Run Launcher

The Multiplayer launcher, downloadable from here.

Man when I enter, it is still the same, no one appears apart from my friend

Not many people play The Simpsons Hit & Run Multiplayer, so that's why.